1. Field of the Invention
The present invention relates to an electronic display formed by forming EL (electroluminescence) elements on a substrate. In particular, the present invention relates to an EL display using semiconductor elements (elements using a semiconductor thin film). Further, the present invention relates to a display device with an EL display used in its display portion.
2. Description of the Related Art
Recently, technology for forming TFTs on a substrate has greatly progressed, and its application to an active matrix electronic display is actively developed. In particular, TFTs using a polysilicon film have higher field effect mobility (also referred to as mobility) than that of conventional TFTs using an amorphous silicon film, and thus, they are capable of high-speed operation, which makes it possible to control pixels with a driver circuit formed on the substrate having the pixels formed thereon, while, conventionally, such control of pixels is performed by a driver circuit provided outside the substrate.
Since various kinds of circuits and elements are formed on one substrate in such an active matrix electronic display, there are various advantages such as reduction in the manufacturing cost, miniaturization of the electronic display, improvement in yield, and improvement in throughput.
In addition, active matrix EL displays having EL elements as light emitting elements are actively researched. EL displays are also referred to as organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
Different from a liquid crystal display, an EL display is of a light emitting type. An EL element is structured such that a layer containing an organic compound which causes luminescence by applying an electric field thereto (hereinafter referred to as an EL layer) is sandwiched between a pair of electrodes (an anode and a cathode). Normally, the EL layer has a laminated structure. A typical laminated structure is “a positive hole transport layer/a light emission layer/an electron transport layer” proposed by Tang et al. of Eastman Kodak Company. This structure has a very high light emission efficiency, and thus, is adopted by almost all EL displays under research and development at present.
The structure may also be such that “a positive hole injection layer/a positive hole transport layer/a light emission layer/an electron transport layer” or “a positive hole injection layer/a positive hole transport layer/a light emission layer/an electron transport layer/an electron injection layer” are laminated in this order on an anode. Further, a fluorescent pigment or the like may be doped into the light emission layer.
In the present specification, all layers provided between a cathode and an anode are collectively referred to as an EL layer. Therefore, all of the above-mentioned positive hole injection layer, positive hole transport layer, light emission layer, electron transport layer, and electron injection layer are included in the EL layer.
When the pair of electrodes apply predetermined voltage to the EL layer structured as in the above, carriers recombine in the light emission layer to emit light. That an EL element emits light is herein referred to as “the EL element is driven”. It is also to be noted that a light emitting element formed of an anode, an EL layer, and a cathode is herein referred to as an EL element.
Light emitted by an EL layer can be broken down into light emitted when a particle returns from a singlet excited state to a ground state (fluorescence) and light emitted when a particle returns from a triplet excited state to a ground state (phosphorescence). In the present invention, either one of the above two kinds of light emission may be used, or alternatively, both of them may be used.
Methods of driving an EL display include an analog driving method (analog driving). An analog-driven EL display is described with reference to FIGS. 26 and 27.
FIG. 26 illustrates a structure of a pixel portion 1800 of an analog-driven EL display. Gate signal lines G1–Gy to which a gate signal from a gate signal line driver circuit is inputted are connected to gate electrodes of TFTs 1801 for switching of the respective pixels. One of a source region and a drain region of each of the TFTs 1801 for switching of each pixel is connected to a source signal line (also referred to as a data signal line) S1, . . . , Sx to which an analog video signal is inputted, while the other is connected to a gate electrode of a TFT 1804 for EL driving of each pixel and to a capacitor 1808 of each pixel.
A source region of the TFT 1804 for EL driving of each pixel is connected to a power supply line V1, . . . , Vx, while a drain region of the TFT 1804 for EL driving is connected to an EL element 1806. Electric potential of the power supply lines V1 to Vx is referred to as power source potential. Further, the power supply lines V1 to Vx are connected to capacitors 1808 of the respective pixels.
The EL element 1806 has an anode, a cathode, and an EL layer provided between the anode and the cathode. In case the anode of the EL element 1806 is connected to the drain region of the TFT 1804 for EL driving, the anode of the EL element 1806 is a pixel electrode while its cathode is an opposing electrode. Conversely, in the case where the cathode of the EL element 1806 is connected to the drain region of the TFT 1804 for EL driving, the anode of the EL element 1806 is an opposing electrode while its cathode is a pixel electrode.
It is to be noted that the electric potential of an opposing electrode is herein referred to as opposing potential, and a power source which applies the opposing potential to an opposing electrode is herein referred to as an opposing power source. The difference between the potential of a pixel electrode and the potential of an opposing electrode is voltage for EL driving, which is applied to the EL layer.
FIG. 27 illustrates a timing chart in the case where the EL display illustrated in FIG. 26 is driven in an analog method. A period from the time when one gate signal line is selected to the time when the next gate signal line is selected is referred to as one line period (L). A period from the time when one image is displayed to the time when the next image is displayed is one frame period (F). With regard to the EL display illustrated in FIG. 26, since the number of the gate signal lines is y, y line periods (L1 to Ly) are provided in one frame period.
In the present specification, that a gate signal line is selected means that all the thin film transistors whose gate electrodes are connected to the gate signal line are in the ON state.
As the resolution becomes higher, the number of line periods in one frame period increases, and accordingly, a driver circuit has to be driven at a higher frequency.
First, the power supply lines V1 to Vx are held at a certain power source potential. The opposing potential which is the potential of the opposing electrodes is also held at a certain potential, which has different power source potential such that the EL elements emit light.
In a first line period (L1), the gate signal line G1 is selected according to a gate signal inputted from a gate signal line driver circuit to the gate signal line G1.
Then, an analog video signal is sequentially inputted to the source signal lines S1 to Sx. Since all the TFTs 1801 for switching connected to the gate signal line G1 are in the ON state, the analog video signal inputted to the source signal lines S1 to Sx is inputted through the TFTs 1801 for switching to the gate electrodes of the TFTs 1804 for EL driving.
The amount of electric current through channel forming regions of the TFTs 1804 for EL driving is controlled by the magnitude of the potential (voltage) of the signal inputted to the gate electrodes of the TFTs 1804 for EL driving. Therefore, the potential applied to the pixel electrodes of the EL elements 1806 is determined by the magnitude of the potential of the analog video signal inputted to the gate electrodes of the TFTs 1804 for EL driving. The EL elements 1806 emit light under control of the potential of the analog video signal.
The above-described operation is repeated. When the analog video signal has been inputted to all the source signal lines S1 to Sx, the first line period (L1) ends. It is to be noted that the period inputting of the analog video signal to the source signal lines S1 to Sx and a horizontal retrace line period may be one line period.
Then, in a second line period (L2), the gate signal line G2 is selected by the gate signal. As in the case of the first line period (L1), an analog video signal is sequentially inputted to the source signal lines S1 to Sx.
When the gate signal is inputted to all the gate signal lines G1 to Gy, all the line periods L1 to Ly end. When all the line periods L1 to Ly end, one frame period ends. During one frame period, all the pixels carry out display to form one image. It is to be noted that all the line periods L1 to Ly plus a vertical retrace line period may be one frame period.
As described above, the amount of light emitted by the EL elements 1806 is controlled according to the analog video signal. By controlling the amount of the emitted light, gradation display is carried out. This method is the so-called analog driving method, where gradation display is carried out by changing the potential of the analog video signal inputted to the source signal lines.
The control of the amount of current supplied to the EL elements by the gate voltage of the TFTs for EL driving in the above-described analog driving method will be described in detail with reference to FIG. 28.
FIG. 28A is a graph illustrating the transistor characteristics of the TFT for EL driving. Reference numeral 2801 is referred to as IDS−VGS characteristics (or an IDS−VGS curve), wherein IDS is drain current and VGS is voltage between the gate electrode and the source region (gate voltage). By using this graph, the amount of current with regard to arbitrary gate voltage can be known.
When gradation display is carried out in the analog driving method, a region indicated by a dotted line 2802 of the above-mentioned IDS−VGS characteristics is used to drive the EL element. FIG. 28B is an enlarged view of the region surrounded by the dotted line 2802.
In FIG. 28B, a region illustrated by diagonal lines is referred to as a saturated region. More specifically, in the region, the gate voltage satisfies |VGS−VTH|<|VDS|, wherein VTH is threshold voltage. In this region, the drain current changes exponentially as the gate voltage changes. This region is used to perform current control by the gate voltage.
When a TFT for switching is turned on, an analog video signal inputted to a pixel is gate voltage of a TFT for EL driving. Here, according to the IDS−VGS characteristics illustrated in FIG. 28A, drain current with regard to certain gate voltage is decided in a ratio of one to one. More specifically, correspondingly to the voltage of the analog video signal inputted to the gate electrode of the TFT for EL driving, the potential of the drain region is decided. Predetermined drain current passes through the EL element, and the EL element emits light in an amount which corresponds to the amount of current.
As described above, the amount of light emitted from the EL element is controlled by the video signal, and, by controlling the amount of light emission, gradation display is carried out.
However, the above-described analog driving method has a defect in that it is easily affected by variation in the characteristics of the TFTs. Even in the case where equal gate voltage is applied to the TFTs for EL driving of the respective pixels, if there is variation in the IDS−VGS characteristics of the TFTs for EL driving, the same drain current can not be outputted. Further, as is clear from FIG. 28A, since the saturated region where the drain current changes exponentially as the gate voltage changes is used, a slight shift in the IDS−VGS characteristics can result in considerable variation in the amount of outputted current even if equal gate voltage is applied. In this case, slight variation in the IDS−VGS characteristics results in considerable difference in the amount of light emitted from the EL elements between adjacent pixels even if a signal of equal voltage is inputted thereto.
In this way, analog driving is quite sensitive to variation in the characteristics of the TFTs for EL driving, which is an obstacle to gradation display by a conventional active matrix EL display.